Generally, low-melting brazing agents have been used to braze or solder certain low-melting aluminum materials. Low-melting fluxes have also been employed in such brazing or soldering operations so as to remove oxides from the portions where the materials should be adjoined. In manufacture for example of heat exchangers or suction manifolds, cast pieces of a low-melting aluminum have been fixedly adjoined to other parts, by using some low-melting brazing agents composed of an Al--Si--Zn alloy. As for the fluxes, chloride fluxes such as KCl--NaCl--LiCl flux and KCl--NaCl--BaCl.sub.2 flux as well as fluoride fluxes have been used in the low-temperature brazing. A reactive flux composed mainly of ZnCl.sub.2 has a melting point lower than the fluxes exemplified just above, and has been used sometimes in the low-temperature soldering. Those prior art fluxes used in combination with the low-melting brazing agent are all problematic as will be detailed below since the melting point of the Al--Si--Zn alloy used in brazing said aluminum cast pieces is 530.degree.-560.degree. C.
KCl--NaCl--LiCl alloy fluxes included in the chloride fluxes are comparatively active but so hygroscopic that the muffles, mesh belts and tools or devices are likely to be seriously damaged within an oven filled with N.sub.2 gas in which the brazing is conducted. On the other hand, KCl--NaCl--BaCl.sub.2 fluxes which are not so hygroscopic are suited for use in the brazing carried out within the oven. However, their activity is comparatively low and their melting point of about 540.degree. C. is undesirably high, so that they are not proper fluxes for use in the low-temperature brazing.
The fluoride fluxes tend to react with Mg present in the aluminum alloy articles to be brazed, thereby producing certain compounds of Mg--F series. If such Mg--F compounds are left unremoved, then the quality of the brazed aluminum articles will be affected to a significant extent. Thus, the fluoride fluxes must be used only to braze low-Mg cast pieces which contain 0.5% or less by weight of Mg.
The reactive fluxes which are composed mainly of ZnCl.sub.2 will react with aluminum element constituting the aluminum articles. A large amount of harmful gases produced through the reaction will corrode the muffles, mesh belts and tools in the oven. A temperature at which such a reaction takes place is low and about 400.degree. C. Consequently, the reactive fluxes are not employable in the brazing process using the low-melting Al--Si--Zn brazing agent.
Objects of the present invention which was made to resolve these problems are therefore to provide a novel flux which proves its merits when used to assist the low-temperature brazing of low-melting aluminum materials of various chemical compositions, as well as a method of flux brazing the low-melting aluminum materials under the presence of the novel flux.